Brian Peters wrote
It beggars belief that the song would have been known at locations 350 miles apart simply by travelling along some rural grapevine

Last Saturday I sang MacDonald's Return To Glencoe in a folk club; the same twelve verses exactly the lyrics, if not the great ability to put over a song, that I had recorded from the great Davy Stewart in 1972. One of my enduring memories of that meeting with him was asking him to write his name and address for me and handing him my notebook and a pen and my acute embarrassment as the pen hovered over the paper, he said to me, "Ye'd better write it oot yersel'. laddie, I ha'nae got ma specs wi'me." (He was wearing them at the time). I still blush when I think of that 45 years later. He then chanted out his Possil Park, Glasgow address for me.
Afterwards, I thought to myself that I had found something very important, a 12 verse broken token ballad from a man who could not read and write! The oral tradition at work!
Later again I thought of two lines that Davy had sung:-

Like she whom the prize of Mount Ida had won,
There approached a fair lassie as bright as the sun....."

Something about those lines stuck out like a sore thumb. I wondered if some minor poet/broadside hack had been at those lines. They were very different from other ways the start of the story appears in other broken token ballads.

Decades later I was at the Take 6 project was being launched at Cecil Sharp House. When was that? Something like 6 years ago? This was the pilot project that EFDSS had for digitising the collections of all the great collectors. One of the speakers at the launch - Malcolm Taylor? Steve Roud? (more likely) had said that one of the first six collectors in that pilot group had collected songs in Portsmouth Workhouse. My ears pricked up. That old workhouse building was now part of St. Mary's Hospital and I had ridden past it on my bike every day in the seven years of my secondary education. As soon as I got home I did an internet search for George Gardiner + Portsmouth and it appears that this had been something of a treasure trove of old songs, but what was this? To my surprise there was MacDonald's Return To Glencoe notated in the first few years of the 20th century and give or take a few words it was identical to the version that I had recorded from the man whose by-name amongst the Scots Travellers was 'the Galoot'.
The Portsmouth version also had 12 verses and included the lines I quote above.

My interest in the song re-kindled by singing it for the first time in while in public for quite a few years, I did an internet search for it. One of the references that the search found was from The County Clare Library. Now for those of you who don't know, one of the things that this library does is to share on its website/database the material collected by a number of important song collectors in the west of Ireland. My link takes you to an article, The Long Song Singer: Martin Reidy of Tullaghaboy 1901-1985 by Tom Munnelly.
Tom writes about Martin

His spartan cottage is just off the road from Connolly to Lisroe in West Clare. In this cottage he was born and reared. He spent all his long life there, a solitary bachelor eking out a living on his mountain farm after his parents had departed this world and the other members of the family had scattered to the four winds. Not that Martin was discontented with such a life, for he had little inclination to travel beyond his immediate environs except perhaps to walk his cattle to the fairs in Ennis or maybe go for a pint and do some shopping in Connolly. His disinterest in the world beyond his mountain was such that he never even travelled the twenty-odd miles to the mating Mecca of Lisdoonvarna in all his years.

Tom goes on to quote that same song in the version he collected from this isolated informant.
Tom only got nine verses from Martin (Vic writes showing his smug side) but they again include the two lines that I quote above. This makes me share the opinion expressed by the worthy Mr. Peters that this "beggars belief" that this travelled between these three different locations in time and location without the aid of print.
I go to the Roud Broadside Index and search for this song and find quite a number of references to it.